This application relates to an efficient and effective method of recovering oil, and ensuring high viscosity oil for a refrigerant compressor.
In the prior art, refrigerant cycles typically include a compressor delivering compressed refrigerant to a condenser. From the condenser, the refrigerant travels to an expansion valve, and then to an evaporator. From the evaporator, the refrigerant returns to the compressor to be compressed.
The compressor is typically provided with lubricant, such as oil, which is utilized to lubricate bearing and other running surfaces. The oil mixes with the refrigerant, such that the refrigerant leaving the compressor includes a good quantity of oil. This is somewhat undesirable, as in the closed refrigerant system, it can sometimes become difficult to maintain an adequate supply of lubricant to lubricate the compressor surfaces. In the past, oil separators have been utilized immediately downstream of the compressor. While oil separators do separate the oil, they have not always provided fully satisfactory results. As an example, the oil removed from such a separator will be at a high pressure, and may have an appreciable amount of refrigerant still mixed in with the oil. This lowers the viscosity of the oil. The use of a separator can also cause a pressure drop in the compressed refrigerant, which is also undesirable.
Further, electric heaters have been utilized to vaporize the liquid refrigerant from the oil. However, the use of an electric heater has energy costs that are somewhat undesirable.
In some proposed systems, the combined lubricant and oil has been exposed to a concentrator or vaporizer for boiling off the liquid refrigerant from the oil. In the proposed system, a portion of the liquid refrigerant leaving the condenser passes through the concentrator and is brought into a heat transfer relationship with the combined liquid refrigerant/oil mixture. The refrigerant from the condenser is intended to cause the liquid refrigerant to evaporate and thus xe2x80x9cboilxe2x80x9d out of the combined liquid refrigerant/oil mixture.
This system is not as effective as it could be because it relied upon a refrigerant tapped from the condenser which was for the most part liquid. The cooling that occurred in the concentrator was thus sensible cooling (a non-phase change cooling). Hence, the temperature of the warmer refrigerant/oil mixture approaches the temperature of the xe2x80x9ccoolxe2x80x9d refrigerant tapped from the condenser. This results in a lower average temperature for the heat exchanger, and thus less effective boiling of refrigerant/oil mixture.
In a disclosed embodiment of this invention, compressed gaseous refrigerant is tapped preferably upstream of the condenser and passed into an oil reclaim vaporizer. Preferably, this invention is included for use with a screw compressor. This refrigerant is at a much higher temperature than in the prior art, and thus efficiently boils the refrigerant out of the liquid refrigerant/oil mixture. Also, since the refrigerant is generally gaseous, use can be made of the latent heat of condensation to provide a larger average temperature difference between the heat source and the refrigerant/oil mixture. Stated another way, the compressed gas is condensed in the vaporizer from a gas to a liquid. Rather than just being cooled to a lower temperature to extract heat, it is condensed at a nearly constant temperature. Preferably, an orifice or other flow control device is positioned in a return line downstream of the vaporizer for this tapped refrigerant. The orifice causes a nearly constant pressure as the tapped refrigerant flows through the vaporizer, resulting in a higher average temperature difference between the heat source tapped refrigerant and the oil/refrigerant mixture. Thus, this method more efficiently boils out the refrigerant. The latent heat capacity of the tapped compressed gas is between one and two orders of magnitude higher than that available by sensibly cooling the refrigerant in the liquid state as was the case in the prior art. The heat transfer coefficients associated with condensation are much higher than those associated with sensible (non-phase change) cooling. Hence, this invention is far more effective at boiling off the excess refrigerant from the mixture. With regard to this feature, it should be understood that while the tapped refrigerant is preferably as high a percentage of gas as possible, it is always possible that some liquid might also be entrained. Thus, when this application speaks of a tapped, compressed gas, it should not be understood that the tapped refrigerant need not be entirely gas.
In one preferred embodiment, the refrigerant is tapped immediately downstream of the compressor. In a second embodiment, the refrigerant is tapped within one of the last compression chambers or closed lobes of a screw compressor.
In at least some possible embodiments, refrigerant could be tapped from the condenser, as long as it was tapped from a point in the condenser at which the refrigerant is still at a compressed pressure, and still has a very high percentage of gas. In any of these embodiments, in the vaporizer, the tapped refrigerant is physically separated from the refrigerant/oil mixture.
To provide further heat to boil off a portion of refrigerant, oil delivered to the compressor bearings is heated in the compressor, and returned directly to an oil sump to further boil off refrigerant. Prior to entering the bearings, this oil passes through an orifice, where its pressure is reduced. This process causes a portion of the liquid refrigerant mixed with the oil to flash to a vapor state, further enhancing the viscosity of the oil delivered to the bearings. This oil is heated as it cools the bearings, and the warmed oil is used to further boil off refrigerant. The oil is taken from this sump and returned to the compressor for lubricating the compressor surfaces.
The basic system outlined above has advantages over the prior art in that the separated oil is at a low pressure associated with the evaporator. Oil/refrigerant mixtures at low pressures are generally at higher viscosity than mixtures in prior systems using separators. In such systems, the oil will be at a high pressure. Further, the use of heated refrigerant gas from the compressor ensures more efficient boiling off of the refrigerant than the prior art.
In further features, there may be a restriction or a valve on a line leading from the evaporator to control the flow of the liquid refrigerant/oil being sent into the still or evaporator. Also, there may be plural, separately controlled lines from the evaporator to control the combined flow.